浙江龙泉岩群新元古代-早古生代变沉积岩地球化学特征及其对华南构造演化的指示

Geochemical characteristics of Neoproterozoic-Early Paleozoic meta-sedimentary of Longquan Group and implications for tectonic evolution in South China

华夏地块是华南大陆的重要组成部分,但其属于古陆还是造山带一直存在争议。选取华夏地块北部浙南地区龙泉岩群变沉积岩,开展了野外调查、年代学研究、原岩恢复和原岩构造环境判别。龙泉岩群变沉积岩的碎屑锆石年龄谱具有多峰值特征,存在明显的Grenville期和泛非期峰值,反映了其源区与冈瓦纳古陆的亲缘性;4件变沉积岩样品的最大沉积年龄分别为618 Ma、620 Ma、570 Ma和549 Ma,结合区内加里东期花岗岩、变质作用等信息,推测变沉积岩的原岩应形成于新元古代—早古生代。岩相学及地球化学特征分析表明,龙泉岩群变沉积岩的原岩为杂砂岩和粘土岩,为类复理石建造,形成于活动大陆边缘环境。龙泉岩群包括类复理石构成的基质和变基性岩、硅质岩、大理岩等构成的岩块,它们形成于不同时代、不同构造环境,最年轻的基质沉积于震旦纪晚期(570~549 Ma)之后,其构造属性应为新元古代—早古生代俯冲增生杂岩,指示该时期扬子与华夏两大块体尚未碰撞拼贴。

Cathaysia is an important part of the South China block,and whether its tectonic setting is an old continent or orogeny has been controversial.We dated detrital zircon U-Pb ages of Longquan Group meta-sedimentary rocks from southern Zhejiang Province in northern Cathaysia,analyzed the petrography,geochemistry and original tectonic settings of their protolith.The age spectrum of detrital zircons of these meta-sedimentary rocks shows multiple peaks,including the Pan-Africa and the Grenville peaks,reflecting their affinity with the Gondwana paleocontinent.The maximum sedimentary ages of the four samples are 618 Ma,620 Ma,570 Ma and 549 Ma respectively,combined with Caledonian granite intrusion and metamorphism in the region,the protoliths of meta-sedimentary rocks were inferred to be formed in Neoproterozoic to early Paleozoic.According to their petrographic and geochemical characteristics,the protoliths of the meta-sedimentary rocks are graywacke and clay rocks,which belong to flysch-like formation in active continent margin.Longquan Group is constituted by flysch-like matrix and exotic blocks,including meta-basites,siliceous rocks and marbles,formed in different time and different tectonic setting,and the youngest matrix deposited after Late Sinian(570~549 Ma).It is concluded that Longquan Group is Neoproterozoic-early Paleozoic subduction-accretionary complex,which implied that the Cathaysia Block and the Yangtze Block had not collided in this period.